Shoujo-ai is just an unnecessary Western invention. In Japan, a manga is either GL(yuri), or it isn't. This particular series is serialized in Comic Yuri Hime, a through and through yuri anthology.
Most of the times, stuff people call 'shoujo-ai' or 'subtext' are series published in moe-rich seinen magazines such as Comic Cune, and never have any GL-related label in the first place. Of course you won't find any romance reading them.